Glass has been chosen as a substrate in liquid crystal display devices for several reasons: (1) it is transparent; (2) it can withstand the chemical and physical conditions to which it is exposed during display processing; and (3) it can be manufactured at reasonable cost in thin sheets with precisely controlled dimensions. Liquid crystal displays are passive displays which are dependent upon external sources of light for illumination. They are fabricated as segmented displays or in one of two basic matrix configurations. The substrate needs of the two types differ. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second type is extrinsic matrix or active matrix addressed, in which an array of diodes, metal-insulator-metal devices or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both designs, however, two sheets of glass form the structure of the display.
Intrinsically addressed liquid crystal displays are fabricated employing thin film deposition at temperatures of about 350.degree. C., followed by photolithographic patterning. Because of the low temperature requirements involved in the process, soda lime silicate glass having a silica barrier layer thereon to prevent migration of Na+ ions has been used extensively as substrates therefor. A higher performance version of intrinsically addressed liquid crystal displays, termed the super twisted nematic, has an added substrate requirement of extremely precise flatness. That requirement has demanded that the soda lime silicate glasses employed in the displays be polished. Alternatively, Corning Code 7059 glass, a barium boroaluminosilicate glass marketed by Corning Incorporated, Corning, N.Y., which is precision formed into sheet requiring no surface polishing utilizing the downdraw fusion pipe, such as is described in U.S. Pat. Nos. 3,338,696 (Dockerty) and 3,682,609 (Dockerty) has been employed.
Extrinsically addressed liquid crystal displays can be subdivided into two categories: the first based upon metal-insulator-metal or amorphous silicon (a-Si) devices; and the second based upon polycrystalline silicon (poly-Si) devices. Devices formed from poly-Si are processed at substantially higher temperatures than those employed with a-Si thin film transistors. Those temperatures have demanded the use of glasses exhibiting higher strain points than soda lime silicate glasses and Corning Code 7059 glass to preclude thermal deformation of the sheet during processing.
The lower the strain point of the glass, the greater this dimensional change. One means for correcting this problem is to anneal the glass sheet after forming; a process adding significant cost. A more economical solution to that problem is to design glasses with high strain points so the dimensional change is minimal during device processing at about 600.degree. C.
Contamination of thin film transistors by sodium migrating from the glass substrate is a major concern during processing. That problem has led to the use of a coating on the substrate glass to provide a barrier to the migration of the alkali.
Therefore, the principal objective of the present invention was to devise glass compositions operable as substrates in liquid crystal display devices utilizing poly-Si thin film transistors, the glass compositions being essentially free from alkali metal oxides, being relatively inert to the chemicals used in display processing, having a strain point higher than 625.degree. C., and, most vitally, exhibiting long term stability against devitrification when in contact with platinum metal and high temperature refractory ceramic materials utilized in the downdraw fusion pipe referred to above for precision forming thin glass sheeting.